Several fields of industry require the grading of the cut-up lumpy solid materials, granulated and similar materials according to grain size in order to prevent further disintegration of the grains in the course of separation. In several cases a cleaning task, such as removal of the impurities, e.g. waste wood, textile fibers, pieces of paper, coarse grains sticking together after cutting up from the solid mass of material coming out of the drier, prior to further processing, possibly before packing, is required. In this case generally there is no need for grading.
For these purposes various types of sieves are used with a variety of disadvantages.
One of the frequently used sieve systems employs a screening cylinder where the sieve frame is fastened to the cylindrical self-carrying framework. The geometrical longitudinal axis of the cylinder is at a slight dip to the horizontal, and the cylinder rotates around this axis. The mass of material to be sieved is carried into the cylinder, passing in the direction of the slope toward the outlet orifice, while part of its falls through the sieve.
Another known type of rotary screening drum is the prismatic sieve, the cross section of which is polygonal (e.g. hexagonal, or octogonal), otherwise it is identical with the above-mentioned screening cylinder. A disadvantage of such screening drums is that only 1/4th to 1/6th of the surface is operative; thus the specific output is low; the material to be graded is arranged in the drum at an angle, consequently higher power is required for rotation of the drum; at too low a speed the layer of material is too thick, while at too high a speed the grains get squeezed into the sieve apertures, thereby causing clogging.